1. Field of the Invention
The present invention relates to a closing device for molds for the manufacture of articles of thermoplastic material, particularly for blow molding of hollow articles in blow molding machines. The device includes two die plates which are linearly displaceable on guide beams and can be moved by means of a power drive, for example, a synchronizing unit in opposite directions relative to each other. The device further includes a frame with at least one tension rod, a compression rod and two yokes connecting the ends of the rods. The tension rod and the compression rod extend parallel and the yokes extend transversely of the direction of displacement of the die plates. One yoke has a cantilever arm with which it acts directly on one of the die plates. The other yoke is connected to the second die plate with the intermediate arrangement of the power drive.
2. Description of the Prior Art
A closing device of the above-described type is known from U.S. Pat. No. 4 248 582. The device has four beams which are arranged underneath the mold and are located symmetrically relative to a vertical center plane extending in closing direction. The lines of application of the closing forces supplied by the power drive coincide with this vertical center plane. The four beams are arranged in two horizontal planes one above the other, however, the die plates are guided only on the beams in the upper horizontal plane. The lower beams of this closing device act as support elements for devices which are used for moving the entire closing device from an extruder station into the blow station and vice versa.
By arranging a synchronizing unit with the two die plates, the known closing device ensures that the existing power drive or closing motor moves the two die plates or the two mold halves connected to the die plates exactly toward each other and apart from each other by exactly half the travel path. However, this known closing device has the disadvantage that the closing forces acting on the mold halves through the two die plates result in deformations of the elements transmitting the forces, particularly of the guide beams for the die plates, so that the closing position of the two mold halves and the manner of operation of the mold halves may be substantially impaired.
European patent No. 0 095 008 discloses a closing device in which the closing pressure for the two mold halves is to be made uniform even in the case of large molds by using weak beams and closing motors. In addition, special measures are taken for preventing undesirable deformations of the elements which transmit the closing force. In this device, three beams are provided which extend through the two die plates. Two of these beams are located in an upper horizontal plane and symmetrically relative to a vertical plane, while the third beam extends along the line of intersection of the horizontal plane and the vertical plane. Moreover, a connection which is pivotable about a horizontal axis is provided between one of the die plates and the corresponding mold half.
However, practical experience has shown that all these measures suggested in European patent No. 0,095,008 for preventing the above-described deformations are not successful. This is because it has been found that the die plates having the pivotable connection with one of the mold halves cannot meet the requirements with respect to being free of deformation in all respects because this die plate not only has to operate as a guide but must also apply the closing force. Accordingly, the guide beams are subjected to bending forces.
It is, therefore, the primary object of the present invention to improve a closing device of the above-described type, so that the guidance of the die plates and the means for applying the closing force are uncoupled from each other in an efficient and space-saving manner.